FIG. 46 is a circuit diagram showing a first known example of the above type of power conversion apparatus. This apparatus includes a single-phase ac power supply 101, reactor 102, converter 201 for shaping the waveform of input current into a sinusoidal wave having a high power factor, smoothing capacitor 202 provided in a dc intermediate circuit, and a three-phase voltage type inverter 231 for driving an induction motor 501 at variable speeds. In FIG. 46, the induction motor 501 is represented by its equivalent circuit.
In the converter 201 shown in FIG. 46, an ac power supply voltage applied through the reactor 102 is short-circuited by semiconductor switches, to thus form a suitable waveform of input current. As a result, ac power generated from the ac power supply 101 is converted into dc power, and the waveform of the input current is controlled to be in the shape of a sinusoidal wave. On the other hand, the inverter 231 consists of a three-phase voltage type PWM inverter, or the like, which includes three pairs of upper and lower arms each consisting of a self-commutated semiconductor switching element, such as IGBT, and a diode that is connected in reverse parallel to the switching element. The operation of this three-phase voltage type PWM inverter is well-known in the art, and therefore will not be explained in detail. The inverter 231 may operate in a selected one of six switching patterns established by controlling the conduction states of the six arms so as to control voltage between respective lines of the three phases, and two switching patterns associated with a zero-voltage vector, which are established by conducting all of the upper arms or all of the lower arms, so that all of the voltages between the lines of the three phases are made equal to zero level.
In the following description of other known circuits, the same reference numerals as used in FIG. 46 will be used for identifying functionally corresponding components or elements.
FIG. 47 is a circuit diagram showing a second known example of the above-described type of power conversion apparatus. The apparatus includes a dc power supply 103, and a converter (two-quadrant chopper) 204 which consists of one pair of upper and lower arms and serves to control the voltage applied to the inverter 231.
In this known circuit, dc power supply voltage applied through the reactor 102 is short-circuited by semiconductor switches, so that some energy is stored in the reactor 102. When the semiconductor switches are turned off, the energy of the reactor 102 is supplied, together with energy from the dc power supply 103, to the smoothing capacitor 202, so that the dc voltage of the smoothing capacitor 202 becomes higher than the power supply voltage.
In the power conversion apparatuses shown in FIG. 46 and FIG. 47, the capacitance of the smoothing capacitor 202 is made sufficiently large, so that switching operations of the converter 201 or converter 204, and the inverter 231 can be freely performed independently of each other.
FIG. 48 is a circuit diagram showing a third known example of the above type of power conversion apparatus, wherein reference numeral 104 denotes a single-phase, full-wave rectifier circuit consisting of a diode bridge, and reference numeral 205 denotes a converter in which the upper arm consists solely of a diode.
In the apparatus shown in FIG. 48, ac power supply voltage is subjected to full-wave rectification by the full-wave rectifier circuit 104, and the resulting dc voltage applied through the reactor 102 is short-circuited by semiconductor switches, thereby to form a suitable waveform of input current. In this manner, ac power generated from the ac power supply 101 can be converted into dc power, and the waveform of the input current can be controlled to be in the shape of a sinusoidal wave.
FIG. 49 is a circuit diagram showing a fourth known example of the above type of power conversion apparatus. This circuit diagram is disclosed in a paper titled "715 Reduction in Capacitance of Capacitor of Single-phase PWM Converter Having DC Active Filter Function" printed in 1996 National Convention Record I.E.E. Japan.
The apparatus shown in FIG. 49 includes a single-phase ac power supply 101, reactor 102, converter 201, inverter 231, two-quadrant chopper 401, smoothing capacitor 202 provided in a dc intermediate circuit, reactor 403 and capacitor 404 used for filters, and an induction motor 501.
While the operation of this circuit will not be described in detail, its basic operation is such that the converter 201 performs PWM control so as to keep a sinusoidal waveform of ac input current, while controlling the input power factor to 1. In order to absorb power ripple arising at the dc output side of the converter 201 and having a frequency that is twice as high as the power supply frequency, the two-quadrant chopper 401 controls the voltage of the capacitor 404 so as to supply and receive energy, thereby to reduce the capacitance of the smoothing capacitor 202.
FIG. 50 is a circuit diagram showing a fifth known example of the above-described type of power conversion apparatus. This circuit diagram is disclosed in a paper titled "One Measure to Reduce DC Voltage Ripple of Single-phase PWM Converter" printed in the Transactions of I.E.E. J. A Publication of Industry Applications Society published in 1993 (vol. 113-D, No. 9, p. 1106-p. 1107).
FIG. 51 is a circuit diagram showing a sixth known example of the above type of power conversion apparatus. This circuit diagram is disclosed in a paper titled "79 Method for Reducing Power Ripple of Single-phase Voltage Type PWM Converter" printed in 1996 National Convention Record I.E.E.J. Industry Applications Society.
In FIG. 50, reference numeral 405 denotes a LC filter in the form of a series resonance circuit that is coupled to a dc intermediate circuit. In FIG. 51, reference numeral 406 denotes a reactor.
While the operations of these circuits will not be described in detail, their basic operations are such that power ripple arising at the dc output side of the converter 201 and having a frequency that is twice as high as the power supply is absorbed by the LC filter 405 of FIG. 50 or the reactor 406 of FIG. 51 having the same resonance frequency, so that the capacitance of the smoothing capacitor 202 can be reduced.
In any case of the known circuits shown in FIG. 46 through FIG. 51, the reactor 102 needs to be provided on the input side of the converter 201, 204 or 205, for the purpose of absorbing ripple that arises upon switching of the converter, and therefore the overall size and cost of the power conversion apparatus cannot be reduced as desired.
In the known circuits shown in FIG. 49 through FIG. 51, the reactor (reactor of the LC filter 405 or reactor 406) is used for absorbing power ripple, and therefore the size and cost of the power conversion apparatus cannot be reduced as desired.
In the known circuits shown in FIG. 49 and FIG. 51, one pair of upper and lower arm (two-quadrant chopper 401) needs to be added to the dc intermediate circuit, and therefore the size and cost of the power conversion apparatus cannot be reduced as desired. Also, the known circuit as shown in FIG. 50 suffers from a problem that the breakdown voltage of the capacitor of the LC filter 405 becomes twice as high as the intermediate dc voltage.